The aim of the proposed research is to measure extracellular Ca2+ depletion in ventricular heart muscle from frog and cat and relate it to the force development and the membrane current. Such measurements will give information about the different components of sarcolemmal Ca2+ transport and the relative importance of the different sources of activator Ca2+. In this project Ca2+ sensitive dyes are used to measure changes in the extracellular Ca2+ content during single action potentials and voltage clamp depolarizations. The optical measurements of extracellular dye absorption are performed in a single sucrose gap chamber equipped also with an isometric force transducer and a voltage clamp system: 1) The effects of drugs and ionic interventions (epinephrine, Ca2+ channel blockers, digitalis, low Ko, low Nao, low Clo) on the Ca2+ signal in frog ventricular muscle will be related to simultaneous changes in the twitch force and the action potential. 2) Voltage clamp experiments will be used in an attempt to quantify the properties of sarcolemmal Ca2+ transport mechanisms. Specific drugs will be used to suppress or enhance either the slow inward Ca current or coupled transport mechanisms (Na/Ca exchange). 3) The involvement of internal Ca stores in excitation-contraction coupling will be examined by measuring the extracellular Ca depletion which persists between beats. In frog and cat it will be examined if Ca is released from such stores during subsequent beats or voltage clamp depolarizations. This project may contribute to the basic understanding of excitation-contraction coupling in heart muscle and elucidate Ca2+ transport mechanisms mediating the actions of cardiac drugs.